Structure and superconducting properties of pure and variously doped bulk MgB2 obtained by uniaxial and isostatic hot pressing

Abstract

The preparation of dense MgB2 bodies, undoped and doped with different atomic species (Na, Ag, Y), was performed via reactive sintering by uniaxial and isostatic hot pressing, starting from the pure elements, and compared with undoped samples obtained by commercial MgB2 powder. The superconducting characteristics of the obtained materials, namely critical temperature (Tc) and current (Jc), were obtained through ac susceptibility measurements and compared to their structural features, like phase purity and secondary phases formation and distribution in the MgB2 matrix. Both the adopted hot pressing techniques gave rise to undoped MgB2 pieces exhibiting phase purity in the range 85–95% and relative density above 80%; although in most cases the doped samples underwent higher extents of phase decomposition and lesser densification, they all exhibited higher critical temperature and current compared to the corresponding undoped material, indicating a net influence of the doping on the superconducting behaviour of MgB2, particularly Ag and Y. An opportune quality factor was adopted, to obtain a more reliable comparison between the different MgB2 samples and evaluation of the samples goodness, in relation to their superconducting characteristics. It was put in evidence that low amounts of doping can improve the superconducting behaviour of MgB2 and that this influence can be addressed in terms of pinning centres, as there was no experimental evidence of an actual atomic substitutions in the MgB2 crystal.